The present invention relates to apparatus for projecting a screen pattern onto a moving photoconductor of a duplicator or the like and, more particularly, to apparatus for projecting a dot screen pattern using a point light source.
The use of screen patterns and screening processes to improve the rendition of images is well known in the fields of photography and electrography. See, for example, U.S. Pat. No. 3,493,381, which issued in the name of R. E. Maurer on Feb. 3, 1970, and U.S. Pat. No. 2,598,732, which issued in the name of L. E. Walkup on June 3, 1952. These patents also disclose a variety of screen patterns including circular or rectangular dots, checkerboard patterns, lines, etc.
U.S. Pat. No. 4,227,795, which issued in the name of R. M. Bobbe et al on Oct. 14, 1980, is another example of the use of a screening process in multicolor electrography. As indicated in the latter patent, the screening process may be carried out by transmitting a light image of the original document through a screen to expose a charged photoconductive member or, alternatively, an image of a screen pattern can be formed on the charged photoconductor either before or after the formation of the image of the original document to be copied by flashing a lamp adjacent the screen. Such patent also discloses dot screen patterns. Because the dot screen is stationary and the photoconductor is moving, a timing mechanism is provided to flash the lamp for the screen at the proper time in a cycle of operation.
U.S. Pat. No. 3,535,036 discloses the use of either a point light source or an elongate lamp in half-tone screening apparatus. Such apparatus requires one or more lens elements between the light source and the photoconductor.
It is also known to transmit a screen pattern through a transparent charged photoconductor. In this regard, reference is made to Item 18542 on page 500 of the September, 1979 edition of Research Disclosure, published by Industrial Opportunities, Ltd., Homewell, Avant Hampshire, PO9-1EF, United Kingdom. Such publication discloses the formation of a half-tone illumination pattern on a transparent photoconductive belt by using a roller positioned in contact with the inside surface of the belt. The roller comprises an opaque cylindrical shell which engages the belt and is driven synchronously with the belt. A suitable pattern, such as a pattern of apertures, is formed on the roller. Located inside the roller is a semi-cylindrical light shield and an elongate cylindrical light source. Rays of light from the source pass through a slot in the light shield onto the roller to thereby project the pattern of apertures onto the belt. Because the belt is transparent, the light pattern selectively discharges the charged, opposite surface of the photoconductive belt. Research Disclosure Item No. 19629 on page 321 of the August, 1980 edition of Research Disclosure discloses a modification of the first mentioned RD publication wherein a multi-colored dot screen is provided for a color copier.
When screen patterns are used the screen is sometimes located closely adjacent to the photoconductor and therefore may become contaminated, for example by particles of toner from the photoconductor. Accordingly, apparatus can be provided for cleaning such particles from the screen. In this regard, reference is made to U.S. Pat. Nos. 3,961,848 and 3,958,877.